What is a brain tumor? A brain tumor is a collection of abnormal "cells" in the brain which are growing out of control. These cells were originally normal brain cells. For no good reason, they started to divide and make more of themselves. This growing collection of cells is called a "tumor." As the cells continue to make more of themselves, the "tumor"gets bigger and bigger, like bread baking. Since there is not a lot of extra room in the brain, the tumor may start to squish areas of the brain, causing them not to work properly. Answered by Dr. Regina Jakacki, pediatric oncologist, Children's Hospital of Pittsburgh July, 2007